Constant motor speed is maintained by the system of the invention for any particular speed setting in the presence of varying mechanical loads on the motor over a permissible range by cancelling the positive resistance of the motor's rotor. This is achieved by placing a negative resistance in series with the rotor so that the voltage across the inductive impedance of the rotor remains constant for a given speed setting as the mechanical load on the motor varies. The negative resistance is generated by the use of positive and negative feedback in the motor drive amplifier.
Dynamic braking is achieved in the system of the invention by a transistor circuit which is connected to extract energy from the rotor when the rotor is rotating at a speed above a selected level. The dynamic braking circuit extracts energy from the rotor under these conditions until it slows the rotor to the selected rotational speed.
The guardian circuit protects the motor from damage due to sustained overloads, while permitting normal operation in the presence of intermittent overloads of short duration. If an overload is not of sufficient duration or magnitude to cause damage to the motor, the speed control system will continue to supply power to the motor.
The dual action foot and panel control in one embodiment of the invention is connected so that when the panel potentiometer is moved from its minimum position, it takes over the control of the motor, and the foot-operated potentiometer acts only as a switch to turn the motor on or off. In this manner, there is no need for an extra switch to select either the foot-controlled potentiometer or the hand-controlled panel potentiometer.
In another embodiment of the invention, speed is controlled only by the hand-controlled panel potentiometer, and a foot-operated switch is provided whose function is only to turn the system on or off.